A switching-type mirror adjusting reflection and light transmittance by external stimulation has been researched by various methods. For example, a chromic glass where transmittance can be autonomously adjusted is being researched and developed. Examples of a kind of transmittance variable glass include electrochromic, photochromic, thermochromic, SPD (suspended particle device), and liquid crystal glasses, and the like. In an active transmittance switchable glass, transmittance can be artificially adjusted by applying electricity in an electrochromic, liquid crystal, or SPD mode. Accordingly, the active transmittance switchable glass may be applied to a smart window to adjust an internal temperature of a building by sun light that is incident toward the inside and prevent damage to goods by rays and the like. However, the smart window using the active transmittance switchable glass has problems in that stability is reduced according to an increase in temperature by absorbing sun light, and thus the smart window has limited durability, a response speed is slow, and a cost is relatively high.
Therefore, in order to simultaneously reduce sun light and the resultant increase in temperature, blocking of sun light through reflection rather than absorption is required, and to this end, a demand for a smart window which has variable reflectivity, is stable and low-priced, and has high efficiency is increasing. Accordingly, there is an increasing need for an electric mirror device that can be switched between a mirror state and a transparent state, has stable bistability in each state, and can be switched at a low voltage.
This technology can be applied to various kinds of other use of adjusting light transmission and reflection in addition to the smart window. Examples thereof may include a technology of applying as a dimmer in an eyeglass-type display apparatus by switching a reflection-type and a transmission-type, and a technology of limiting maximum brightness of a strong light source reflected on a rear mirror and a side mirror of transportation means to ensure safety of a driver. A current electrochromic technology includes an electrolyte layer in a light path to cause a reduction in resolution in a section having a refractive index difference and thus cause deterioration of a mirror property.
A light modulation technology through light reflection rather than light absorption is in an attempt by a method such as manufacturing of a photonic crystal structure using a switchable electrochromic material, and shutterization of a mechanical metal mirror, but instantaneous formation and decomposition of a metal mirror layer through electrochemical oxidation and reduction reactions of metal ions is most influential. In the prior art which attempts to use reversible electrodeposition of a metal for light modulation is made, there are demerits in that a deposited material obtained on a transparent substrate exhibits a rough black, grey, or occasionally colored appearance, and poor reflectivity and high light absorbance are exhibited, which become more serious particularly when the deposited material is thick.
A technology of increasing reflectance by adding a white pigment in order to improve a contrast exists in the prior art, and in this connection, there is U.S. Pat. No. 5,056,899 by Warszawski, which relates to a display, but in the case where a reversible electrochemical metal is formed, there is a big demerit in that the metal is fixed in a counter electrode, and therefore it teaches that the technology is unsuitable for a display. The document of the prior art describes that an auxiliary counter electrode reaction is required in order to interrupt metal fixation in a working electrode, which does not cause a pure change in transmission, but low reflectance of the described deposited material is not suitable for adjustable mirrors.
An electrolyte described in the prior art document contains auxiliary oxidation-reduction species (for example, bromide, iodide, or chloride) oxidized (for example, oxidized into bromide, iodide, or chloride) in the counter electrode during a metal deposition period under a high used driving voltage, and the auxiliary oxidation-reduction electrolyte cause extinction of the deposited material on an open circuit by chemical dissolving (for example, 2Ag0+Br2→2AgBr) of the metal deposited material, so that stability is low, and in most cases, metal deposition in the counter electrode is interrupted. For example, in the case of all electrodeposition apparatuses found in the prior arts such as the patent document by Warszawski [when copper or nickel exists in a counter electrode paste, refer to columns 3 and 4]; a document by Duchene, et al., [Electrolytic Display, IEEE Transactions on Electron Devices, Volume ED-26, Number 8, pp. 1243-1245 (1979.8)]; and French Patent No. 2,504,290 (Oct. 22, 1982), a high switching voltage of 1 V or more is used and stability is low.
A paper by Ziegler, et al., [Electrochem. Soc. Proc. Vol. 93-26, p. 353, 1993] describes research on use of a reversible electrodeposited material made of bismuth in an aqueous solution containing halide anions and trivalent bismuth ions, in which a mol concentration ratio of the halide anions to the trivalent bismuth ions is large, for displays. An oxidation reaction of the halide anions serves as a counter electrode reaction where a write voltage of 1.5 V is used. The obtained deposited material had a dark color and reduced reflectance of an ITO surface.
Succeeding reports by Ziegler, et al., ([Electrochem. Soc. Proc. Vol. 94-31 (1994), p. 23] and [Solar Energy Mater. Solar Cells 39 (1995), p. 317]) describe that addition of copper ions to an electrolyte is required to achieve complete extinction of a deposited material. Further, Ziegler, et al., used a counter electrode reaction in addition to metal electrodeposition/dissolving reactions, but did not obtain a mirror deposited material. Accordingly, the aforementioned documents by Ziegler, et al., do not provide teaching relating to an effect of an electrolyte composition on deposition/dissolving speeds and quality of the mirror electrodeposited material.
U.S. Pat. No. 5,880,872 by Udaka, et al., describes that a working electrode of a reversible electrodeposition structure is decomposed, and thus a life-span of the electrode is shortened by a high voltage required to dissolve a metal film deposited on the electrode. Udaka, et al., describes that the aforementioned result can be prevented by adding alkali metal halide (preferably, in an amount which makes a ratio of alkali metal halide to silver halide be 0.5 to 5) to an electrolyte solution of an optical apparatus. However, in the prior art document, a bistable or memory effect is very short. This is because a counter electrode reaction occurs in addition to metal electrodeposition/dissolving reactions. A product of a strong oxidation reaction generated in a counter electrode enables a metal deposited material on the working electrode to be chemically dissolved on an open circuit or to be electrochemically dissolved on a short circuit.
In describing a concept of a reversible electrodeposition light modulation apparatus, Zaromb (S. Zaromb, J. Electrochem. Soc. 109, p. 903, 1962) recognized that a concentration of an electrodeposition metal should be high enough to rapidly perform rapid electrodeposition without consuming surplus metal ions in an electrode but should be sufficiently lower than a solubility limitation in order to prevent precipitation during a rapid electrolysis period of a metal deposited material. In the case of a Zaromb's apparatus including electrodeposition of a nephelinite Ag deposited material, an aqueous electrolyte containing AgI in a mol concentration range of 3 to 3.5 M (solubility limitation: 4 M) is recommended, and addition of 7 M NaI is recommended in order to improve conductivity of an electrolyte. Nevertheless, electrodeposition metal ions at a relatively low concentration were used in succeeding work on the reversible electrodeposition light modulation apparatus using a water-insoluble solvent. The reason is because, generally, solubility of the ionic salt is significantly low in the water-insoluble solvent having a dielectric constant that is lower than that of water. Further, the ionic salt at a high concentration in the water-insoluble solvent causes meaningful ion pairing, and the ion pairing may reduce conductivity of the electrolyte and reduce a speed at which a deposited material having high quality can be electrodeposited.
The claims of U.S. Pat. No. 5,880,872 by Udaka, et al., describe use of surplus halides added as a Li, Na, or K salt (0.5 times to 5 times a concentration of silver halide) for assisting in dissolving silver halides for an optical apparatus, but the specification describes only dissolving 0.5 M AgBr in a water-insoluble dimethyl sulfoxide (DMSO) solvent. Likewise, U.S. Pat. Nos. 5,764,401 and 5,864,420 by Udaka, et al., describe only use of 0.5 M AgI or AgBr in DMSO and dimethylformamide (DMF) solvents. In the case of an apparatus by Udaka, et al., even an electric potential of 1 V provided only a current of about 1 mA/cm2. An apparatus having a mirror deposited material, excellent electrolyte stability, bistability, electrochemical stability, or a long cycle life or switching life-span was not obtained even with any electrolyte preparations by Udaka, et al.